Physical activity has been shown to benefit cancer survivors' physical and emotional well-being, however, few studies have focused on the process and determinants of the adoption of physically active lifestyles in cancer survivors populations. The goal of the project is to study predictors of adherence to physical activity in sedentary endometrial cancer survivors who receive an intervention to increase their physical activity. The specific aims of the study are (1) To test a Social Cognitive Theory-based model of physical activity adoption among sedentary endometrial cancer survivors who receive an intervention to increase physical activity; (2) to elucidate the role of cardiorespiratory fitness and somatic sensations during physical activity on self-efficacy; (3) to determine whether intervention dose is related to physical activity adherence; and (4) to test the effects of adherence to physical activity on endometrial cancer survivors' quality of life. Two hundred sixty-seven sedentary Stage I-IIIa endometrial cancer survivors will be recruited to participate in this six month study. Participants will complete fitness tests, questionnaires, and cognitive tests every two months to assess functional capacity and efficiency, physical activity, and Social Cognitive Theory-related variables. All participants will receive an intervention to increase their physical activity, consisting of a customized exercise prescription, telephone counseling, and written materials. Results of the study will provide a rigorous test of Social Cognitive Theory as it is applied to physical activity, and will inform the development of effective interventions for cancer survivors.